1. Field of the Invention
The invention relates to inrush current controllers.
2. Description of Prior Art
Mains power rectification circuits use a large value E-cap an electrolyte capacitor (known as bulk-cap). When AC power is first applied to the rectifier circuit, depending on the instantaneous voltage across the mains line, there may be a large charging current (known as inrush current) flowing through the bulk-cap. Such current, if not limited, may reach a few hundred Amperes. Input connections, fuse, power switch, or the rectifiers may be damaged by such high currents.
One way to control the inrush current is to put a resistor in series with the input circuit. In order to limit the inrush current to a safe level, the serial resistance needs to be comparatively large. Such a large resistance is unnecessary during normal operation and wastes power and generates undesirable heat.
An alternative to a fixed serial resistor is to use a negative temperature coefficient resistor known as an NTC resistor. The resistance of an NTC is large at normal ambient temperature but is significantly reduced when hot. This approach does not necessarily solve the inrush current problem. Firstly, the NTC resistor must be kept hot for its resistance to remain reduced, so that heat generation and power loss are unavoidable. Secondly, once heated, the NTC resistor will take a few seconds to cool down. If the AC power is switched on again while the NTC resistor is still warm, the inrush current will not be reduced.
Another alternative is to use a timed switch to shunt the inrush current limiting resistor following an AC turn on. This timed switch is turned OFF after the AC input is disconnected, usually after the voltage of some internal DC power rail has dropped following the AC disconnection. This approach suffers from a similar problem as using an NTC resistor. The switch takes some time to return to an open state so the AC power cannot be reconnected too soon after disconnection. Otherwise, the limiting resistor will still be shunted by the timed switch and the subsequent inrush current will not be limited.